apcu compatibility with PHP master is fixed, but not released yet. Before apcu failed to compile it was the redis extension. That’s also fixed, but probably not released.
Getting 3rd party PHP extensions to compile against PHP master is pretty much a losing battle. They always become compatible eventually, but it’s a shame to lose nightly builds for long periods of time, because one or the other extension is not compatible with current master.
I’ve submitted an alternative solution to this problem here: https://github.com/travis-ci/php-src-builder/pull/26 We ignore extension build failures for the nightly builds. If an extension builds, that’s great. Otherwise we will build PHP without it.